The present invention relates to a method for controlling a fluidized bed gas phase polymerisation reactor in the production of a polymer.
In particular, the method of the invention combines control of reactor production rate, based on feed flow rate(s) of the monomers, with pressure control inside the fluidised bed reactor.
In the prior art WO 00/22489 relates to a system for the on line control of a plant of polypropylene and its copolymers produced in loop reactors, and, optionally, gas-phase reactor(s) with the aid of mathematical models, wherein said system is based on a control matrix comprising controlled variables, constrained controlled variables, manipulated variables, and disturbance variables. It is suggested that such a control system could enable the maximization of production rate and catalyst yield of the producing process. It is noted that a relevant point regarding process control is the fact that, generally, economic aspects require the process to be operated close to plant capacity limits. However, it is also noted that although catalyst flow rate could be manipulated to control the production rate of a reactor, when polymerization heat exceeds the limit of the thermal exchange capacity of the reactor, the objective of increasing production even more should be restricted in favour of thermal stability.
WO 00/22489 mentions that a variable to be controlled may be the pressure of gas phase reactors and indicates that the mathematical model for control of the pressure of gas phase reactors may use the following as input variables: the flow rate of catalyst fed to the reactor(s); the density of reaction medium of the loop reactor(s); the flow rate co-monomer(s) fed to the gas phase reactor; the gas-phase reactor bed level; the flow rate of gas returning from a separation tower.
In the prior art various other proposals have been made for the control of polymerisation reactions.
For example, SU 1281566 discloses that the gas-phase polymerization or copolymerization of α-olefins in the presence of a catalyst, H, and CO2 is controlled by regulating the reactor pressure and by changing the monomer feed rate, and the melt flow index at different loads is controlled by changing the concentration of CO2 and H. The reactor productivity is increased by regulating the particle size of the resulting polymer by a direct proportional change in the concentration of CO2, taking into account the activity of the catalyst.
U.S. Pat. No. 2,964,511 proposes that, in a liquid phase catalytic polymerisation reaction (i.e. polymerisation of ethylene) in a pressurized reaction zone carrying a liquid phase and a vapour phase and to which polymerisation catalyst and monomer are fed continuously in separate streams, control of the reaction comprises sensing the pressure in said reaction zone and regulating the flow of monomer feed to the reactor in compensation for changes of pressure within vapour phase of the reaction zone. Further, a control signal may be produced as a function of the pressure, and the control signal applied to increase the rate at which catalyst is fed to said reaction zone in response to a pressure increase and to decrease the rate at which said catalyst is fed to said reaction zone in response to a pressure decrease, thereby maintaining said pressure within predetermined limits.
U.S. Pat. No. 6,521,805 relates to an isobutene polymerisation process wherein a property P (viscosity or average molecular weight) of the polyisobutene produced is maintained constant. The polymerisation is conducted continuously in a reactor comprising a boiling liquid reaction phase in equilibrium with a gas phase, by continuous introduction into the reactor of a catalyst and of a C4 hydrocarbon feed mixture comprising the monomer, and by continuous withdrawal from the reactor of the liquid reaction phase. The process comprises the determination of a target value V of the partial pressure, PiC4, of the isobutene in the reactor gas phase corresponding to the desired value of the property P, by virtue of an empirical relationship established beforehand between the property P of the polyisobutene produced and PiC4. During the polymerisation, PiC4 is measured and a corrected value of PiC4, (PiC4)c, is calculated and is held constant at around the target value V, by acting on the flow rate Qc of the catalyst and/or on the flow rate Qh of the C4 hydrocarbon feed mixture.
U.S. Pat. No. 6,263,355 relates to a method for controlling a chemical reactor such as a gas-phase reactor using a non-linear predictive control which includes steps for generating a plurality of signals representing a current state of the chemical reactor and reflecting a respective constituent of reactants in the chemical reactor, calculating a future state of the chemical reactor responsive to said plurality of signals and referenced to mass hold-up of the reactants in the chemical reactor, and controlling at least one parameter related to the chemical reactor so as to control the future state of the chemical reactor. It is suggested that a process control method which prioritises set points, e.g. pressure set point, is to be provided.
WO 96/41822 suggests a method for controlling a process for synthesising at least one chemical in a plant including at least one reactor (R) that may be considered as a perfectly mixed reactor, wherein control variables enable the process to be controlled so that a number of variables known as controlled variables and related to the properties of the product and/or the operation of the process match corresponding set values. The method comprises the steps of (a) inputting set values for the controlled variables; (b) using a prediction unit to calculate predictions of the controlled variables on the basis of measurements of the process control variables; (c) using a control unit to calculate set values for the process control variables on the basis of the set values and predictions of the controlled variables; and (d) transmitting set values for the process control variables to process control actuators. For each process control variable, e.g. temperature, flow rate or pressure, to be controlled its actual value may be measured continuously or intermittently and a PID controller used to compare this actual value with a set value.
It will be understood from the above examples from the prior art that a variety of proposals have been made relating to reactor pressure control, including partial pressure control, in polymerisation reactions.
The prior art (see the above-mentioned SU 1281566, U.S. Pat. No. 2,964,511, U.S. Pat. No. 6,521,805, U.S. Pat. No. 6,263,355, WO 96/41822) indicates that, alongside feed flow rate, catalyst flow rate could also be used for pressure control. The prior art (WO 00/22489) may also indicate that catalyst flow rate can be used for control of reactor production rate.
However, the prior art provides no indication as to how control of reactor production rate based on feed flow rate(s) could be combined with pressure control in a fluidized bed gas phase polymerisation reactor.
The inventors have had the insight that there can be provided a practical and simple method for control of a fluidized bed gas phase polymerization reactor in which control of reactor production rate based on feed flow rate(s) is combined with pressure control, without the production rate control competing with the pressure control, that is with the improvement that the production rate control and pressure control do not act separately on the regulation of the monomer feed rates to the GPR.